The present invention relates to a process for forming a patternwise coated powder layer. Particularly, the invention pertains to a process for forming a patternwise coated powder layer, which process is suited to the production of the phosphor screen of a color picture tube.
As a process for forming a patternwise coated powder layer, processes which comprise using powder as a dispersion or mixture thereof wherein powder has been dispersed in a dispersion medium such as water have heretofore been general. In such a case, a uniform dispersion has first been formed, and the dispersion has then been stored in a uniform state and coated to form a patternwise coated powder layer through complicated steps.
Referring to FIGS. 1-A to 1-F in the accompanying drawing, the formation of red, green and blue light-emitting phosphor layers on the inner surface of the face plate of a color picture tube according to one example of such prior art processes will be explained below.
As is shown in FIG. 1-A, a layer (2) is formed by coating the inner surface of the face plate (1) with a uniform mixture of a first color phosphor, for example, a green light-emitting phosphor and a photosensitive resin solution comprising a photosensitive resin, for example, polyvinyl alcohol and ammonium dichromate and then drying the resulting coating. The layer (2) is exposed to ultraviolet light through the apertures of a shadow mask. Here, the ultraviolet-light-irradiated position corresponds to an incident area of electron beams for exciting the phosphor, that is, the position to which the phosphor is to be bound. The photosensitive resin at the exposed area becomes insoluble and thereby the whole layer at the area becomes insoluble. The layer (2) is then washed with a solvent (usually water). Thus, as shown in FIG. 1-B, only the layer (2') which has become insoluble by exposure to ultraviolet light remains on inner surface of the face plate and the other layer is dissolved off. Thereafter, a layer (3) of a mixture of a second color phosphor, for example, a blue light-emitting phosphor and a photosensitive resin is formed by the same operations as described above as shown in FIG. 1-C. The layer (3) is then exposed to ultraviolet light and washed with water. Thus, as shown in FIG. 1-D, an insolubilized layer (3') remains on the inner surface of the face plate. Further, as shown in FIG. 1-E, a layer (4) of a mixture of a third color phosphor, that is, a red color-emitting phosphor and a photosensitive resin is formed by the same operations. The layer (4) is then exposed to ultraviolet light and washed with water. Thus, as shown in FIG. 1-F, an insolubilized layer (4') remains on the inner surface of the face plate.
As is clear from the above explanation, the steps of producing the phosphor screen of a color picture tube are complicated. Further, many times repetition of wet coating, washing with water and drying is required. It gives rise to various technical problems in each step and is very disadvantageous from an economical point of view.
On the other hand, a simple process which comprises coating powder itself is known from Japanese Patent Laid-Open Publication No. 47-7,266 (1972). According to this process, a supporter for a phosphor screen is coated by a photosensitive lacquer layer, the surface of which is sticky before exposure but loses stickiness by exposure, the area of the photosensitive lacquer layer where the phosphor screen is to be formed is exposed, a light-absorbing material powder which may be bound to the unexposed area is coated onto the layer, said exposed area is washed to remove the excess, unbound light-absorbing material, and finally phosphor screens which emit respectively different color lights are provided.
However, the photosensitive lacquer layer used in this process loses stickiness at its once exposed area. Therefore, it is impossible to coat three phosphors onto the same photosensitive lacquer layer successively. Thus, this process is only a process for forming a light-absorbing material layer (a so-called black matrix), and it is not suitable for forming the phosphor screen.
Further, as the photosensitive lacquer used in this process, "Kodak Mordant Cermitax" manufactured by Eastman Kodak Co. is known. However, this photosensitive lacquer has a defect in that its sensitivity is very low. Therefore, a long time is required for exposure and it is very disadvantageous from a commercial point of view. Further, the photosensitive lacquer is insoluble in water and is soluble only in organic solvents. However, the use of organic solvents on a commercial scale is disadvantageous since vapors of many organic solvents are toxic to a human being and are inflammable.